1. Field of the Invention
The present invention relates to photoelectric data card systems in which the cards are transmissivity-coded, and more particularly to improved transmissivity-coded photoelectric data card systems in which novel and inventive means are provided for reducing or eliminating reading errors arising from batch-to-batch variations in the transmissivity of commercially available card stocks, and for reducing reading errors arising from card-to-card code imprinting variations, thus increasing the practically realizable number of reader-distinguishable digit values, and making it possible to produce cards with a minimum of expensive, time consuming hand labor steps, and without using special chemical techniques.
2. Description of the Prior Art
Transmissivity-coded photoelectric data card systems are well known in the prior art.
A transmissivity-coded photoelectric key-card system is shown and described in U.S. Pat. No 3,029,345, issued to David W. Douglas on Apr. 10, 1962. Another such system is shown and described in U.S. Pat. No 3,383,513, issued to the same inventor on May 14, 1968.
Card constructions for transmissivity-coded key-cards are shown and described in U.S. Pat. No. 3,836,754, issued to Frederick D. Toye and Frederick N. Toye on Sept. 17, 1974.
A photoelectric reader for transmissivity-coded key-cards is shown and described in U.S. Pat. No. 3,875,375, issued to Thomas J. Scuitto and David C. Kramer on Apr. 1, 1975.
Actual working embodiments of prior art transmissivity-coded data card systems have been severely limited in the number of digit values, i.e., transmissivity levels, which could be correctly distinguished by their readers in mass produced cards from different production batches, making it necessary to increase the number of digit areas per card, and thus making the associated readers more complex and expensive than contemplated, and failing to realize the full potential of such systems.
In addition to thus increasing the number of digit areas per card above the optimum, workers in the transmissivity-coded data card art have also resorted to manual sampling of large numbers of mass produced cards, in order to find a small number of cards which would operate correctly with a particular reader or small group of readers. There is, of course, no guarantee that any such sampling will produce any particular number of usable cards, or indeed any usable cards at all.
The individual adjustment of a reader or a group of readers to match the transmissivity levels of a group of cards found to be alike in that respect by manual sampling is, of course, as expensive or more expensive than the manual selection of the cards by sampling.
As will be obvious to those having ordinary skill in the art, these cut-and-try expedients, involving considerable hand labor, and the increase of the number of digit areas per card, digit reading pairs per reader, etc., above the optimum, made the prior art transmissivity-coded data card systems in general as expensive or more expensive than the conventional "black-and-white" card systems, even with the added timing tracks and timing bit reading and translating means characteristic of "black-and-white" systems, and thus transmissivity-coded data card systems have not reached their full potential.